Near field communication (NFC) systems are used for short-range communications between an NFC transmitter device, commonly referred to as an NFC reader, and a powered or unpowered NFC receiver device, commonly referred to as a tag. The NFC reader generates a short range magnetic field which couples with an antenna in the NFC tag, and data can be transmitted between the NFC reader and the NFC tag by modulation of this magnetic field. To this end, the NFC tag typically includes transmit/receive circuitry in addition to the antenna.
When an NFC receiver device such as a tag enters a magnetic field, a voltage is generated at the output of the antenna of the tag. The magnitude of this voltage is dependent upon a number of factors, including the strength of the magnetic field, the distance between the source of the magnetic field and the tag, the type of antenna in the tag, and any antenna interface circuitry present in the tag.
The transmit/receive circuitry in the tag can only tolerate a limited positive or negative voltage at its input ports before damage occurs. In normal operation of an NFC tag (for example when the tag is used to receive and respond to a signal from an NFC reader) this may not be a particular problem, but when the tag enters a strong magnetic field, such as may occur in the vicinity of a wireless charger system, there is a risk that the transmit/receive circuitry of the tag could be damaged by high positive or negative voltages that may develop at the output of the tag antenna as a result of the strong magnetic field.
Accordingly, a need exists for a mechanism to protect transmit/receive and other circuitry in an NFC device such as an NFC tag from potentially damaging voltages that may develop when the NFC device enters or is present in a strong magnetic field.